1. Field of the Invention
This invention relates to a process for the production of new, modified polyisocyanates containing urethane, carboxylic acid anhydride and biuret groups by reacting simple starting diisocyanates with certain hydroxycarboxylic acids and, optionally, water; to the compounds obtainable by this process; and to their use, optionally in a form in which they have been blocked with blocking agents for isocyanate groups, as the polyisocyanate component in polyurethane lacquers.
2. Description of the Prior Art
Modified aliphatic diisocyanates, particularly based on hexamethylene diisocyanate, have acquired universal commercial significance for the production of light-stable, weatherproof lacquers with high gloss retention. Non-colored to faintly colored products are required for application in this field, particularly for clear and white-pigmented coatings. In addition, safe application presupposes a minimal content of monomeric diisocyanates which does not increase, even in the event of prolonged storage. Toxicological investigations have shown that safe processing is possible up to a maximal content of 0.7% of monomeric diisocyanate providing the safety measures normally adopted in the application of lacquers are observed. The above-mentioned limit has been accepted in the literature (cf. for example the pamphlet "PUR-Anstrichstoffe" of the Hauptverband der deutschen gewerblichen Berufsgenossenschaft and also "Polyurethane Report" of the Paintmakers Assoc.).
Over the years, many different forms of modification have been developed for the production of polyisocyanates of the type in question. For example, the following modification products of aliphatic diisocyanates have hitherto been successfully used in practice:
polyisocyanates containing uretdione groups produced by partial dimerization of diisocyanates in the presence of special catalysts,
polyisocyanates containing isocyanurate groups produced by partial trimerization of diisocyanates in the presence of special catalysts,
polyisocyanate mixtures containing both isocyanurate and also uretdione groups produced by partial trimerization and, at the same time, partial dimerization of diisocyanates in the presence of special catalysts which accelerate both the trimerization and the dimerization of isocyanate groups,
polyisocyanates containing urethane groups produced by partial reaction of diisocyanates with polyols, normally with triols and
polyisocyanates containing biuret groups produced by partial reaction of diisocyanates with a suitable biuretizing agent.
However, these modified lacquer-grade polyisocyanates which have been successfully used in practice are still in need of improvement in certain respects.
The uretdione and/or isocyanurate polyisocyanates are generally produced using special catalysts (for example phosphorus-containing compounds, tertiary amines or alkali metal carboxylates) which, to terminate the modification reaction at the required degree of modification, are generally destroyed by the addition of a terminator. In most cases, the resulting products (reaction products of the catalyst with the terminator) cannot be separated off from the end product which may cause clouding or unfavorable properties during the subsequent use of the product.
Although it is also possible to use catalysts which lose their effectiveness during the reaction as the temperature increases so that additional terminators are not required when compounds such as these are used, the exact amount of the catalysts required to achieve precisely defined conversions of isocyanate groups is difficult to determine. Further, the decomposition products of the catalysts remain in the product with the possible disadvantages mentioned above.
Although there is no need to use special catalysts in the production of urethane polyisocyanates, the known modified polyisocyanates solely containing urethane groups often show extremely high viscosities which seriously restrict their use (for example for the production of coatings) because heavy dilution with solvents is necessary to obtain suitable viscosities for processing. However, there is at present a distinct trend towards low-solvent or even solvent-free systems due both to industrial hygiene requirements and also to ecological and economic requirements.
State-of-the-art, lacquer-grade polyisocyanates containing biuret groups can be produced by many different methods in which it is also possible to adapt the viscosity of the modified polyisocyanates to the particular application envisaged. However, the modifying agents used for producing the biuret polyisocyanates (biuretizing agents such as water, compounds which give off water or various amines) are attended by special disadvantages which adversely affect either the method of production or the properties of the product.
If water is used as the biuretizing agent, large quantities of an organic solvent are required to convert the normally immiscible components, water and diisocyanate, into a homogeneous mixture suitable for carrying out the biuretizing process. Despite the use of these solvents, insoluble polyureas are often formed which can no longer be dissolved. Accordingly, in addition to the need to remove large quantities of solvents by distillation, an additional filtration step is often necessary.
Processes involving compounds which give off water (for example tertiary butanol) lead on the one hand to the loss of the biuretizing agent (isobutene) which has to be removed at considerable expense. On the other hand, temperatures above 160.degree. C. have to be maintained for prolonged periods in these processes in order to achieve a complete reaction. However, due to the secondary reactions, temperatures as high as these leave the end products with poor color quality.
This also applies when amines are not formed in situ from isocyanates, but instead the excess diisocyanate is directly reacted with the corresponding diamine. However, the high reactivity of the amines to isocyanates unavoidably results in the formation of high molecular weight polyureas. In order to dissolve these polyureas to form biuret groups involves the application of high temperatures with the resulting deterioration in color quality and an increased occurrence of secondary products. Nor can the application of high temperatures be avoided by using special amines which do not form high-melting ureas with the diisocyanate or, where vapor-form amines are used, by avoiding the formation of solid ureas, because these reactions must of necessity be followed by equilibration reactions which only take place at high temperature. In addition to uretdiones and isocyanurates, carbodiimides and reaction products of carbodiimides also occur, adversely affecting the monomer stability of the end product.
Accordingly, an object of the present invention is to provide new modified polyisocyanates which satisfy all the demands made of high-quality lacquer-grade polyisocyanates, i.e. which show in particular excellent color quality and high monomer stability and which, in addition, can be produced easily under moderate conditions.
This object may be achieved by the process according to the invention which is described in detail in the following. In this process, certain starting diisocyanates are reacted with selected hydroxyalkane carboxylic acids or combinations thereof with water to form urethane, carboxylic acid anhydride and biuret groups.
Although the production of modified polyisocyanates by reaction of simple diisocyanates with aqueous solutions of water-soluble carboxylic acids is already known from DE-OS No. 3,228,721, the reaction of the starting diisocyanates with aqueous solutions of the acids mentioned therein leads to undesirable secondary products containing acyl urea groups formed via the intermediate stage of carboxylic acid amide groups. This not only deepens the color of the modified polyisocyanates according to the above-mentioned prior publication, but there is also the serious disadvantage that the modified polyisocyanates according to that prior publication and the secondary products containing acyl urea groups which are present in them are unstable and tend to split off the monomeric diisocyanates on which they are based. This disadvantage does not arise when the selected modifying agents essential to the invention are used. Instead, the process according to the invention which is described in detail hereinafter provides for the production of new, substantially colorless monomer-stable lacquer-grade polyisocyanates.